The present invention concerns piston internal combustion engines, such as used for example in road transport vehicles. At present, there exists in particular two types of piston internal combustion engines, which are the spark ignition engine, or gasoline engine, and the auto-ignition engine also called diesel engine.
These engines use either a two-stroke thermodynamic cycle or, as in the great majority of the cases, a four-stroke cycle. The main parts of such an engine are a cylinder containing a piston effecting a reciprocal movement which is converted into a rotative movement by means of a connecting rod and of a crankshaft. The four strokes of a spark ignition engine will now be briefly explained. The piston sucks up an air-fuel mixture by going back and then compresses it by going forth and the fuel evaporates under the increase of temperature. When the piston comes close to its dead point, an ignition plug ignites the mixture by means of a spark which provokes a sudden rise of temperature and of pressure. The backward motion of the piston permits the combustion gases to expand and the usable work is produced at this moment. Finally, the forward movement of the piston expels the combustion gases. Therefore, the four strokes are the admission, the compression, the expansion and the discharge. The diesel engine uses a comparable principle where the difference resides in the way of introducing the fuel, which, in this case, is directly injected into the compressed and therefore hot air, and flames up then spontaneously.
In both cases, the energy efficiency depends, among others, on the compression volume ratio. The higher it is, the higher is the efficiency. Unfortunately, this compression ratio is limited, in the case of the gasoline engine, by the risk of premature knocking of the mixture and in the case of the diesel engine among others by the necessity to keep an appropriate combustion chamber. Anyway, it is to be noted that for a thermodynamic cycle such as disclosed above, the increase in efficiency becomes weaker and weaker for an equal increase of the compression ratio starting from a value of 10 to 15 for the latter, and, in the case of the diesel engine, the mechanical stresses determine mainly the critical volumetric compression ratio. The documents which have been used to reflect the state of the art are the book of professor A. Houberechts at the Catholic University of Louvain called "La thermodynamique technique" (the technical thermodynamics), volume 2, 4th edition, pages 325 to 405, published by Ceuterick at Leuven in 1976 and the lecture notes "Moteurs a combustion interne" (internal combustion engines) of the year 1981 of professor J. Martin at the U.C.L.